The Global Positioning System (GPS) operated by the United States Department of Defense is composed of about 24 earth-orbiting satellites that contain precise atomic clocks. The satellites, which are programmed from the ground, transmit microwave signals modulated with navigational and timing information. A suitable receiver can calculate its geographical position after receiving and demodulating the microwave signals transmitted by four or more of these satellites. The receiver can also use the satellite signals to determine the time in either the timescale used by the GPS system or the official United States timescale, UTC (USNO, MC). The receiver can then generate an electrical output pulse at a convenient time or times, such as at each whole second.
The receiver uses the measured propagation times of the microwave signals from four or more satellites and the calculated positions of the satellites when the microwave signals were transmitted to calculate its position and the time. Any factor that modifies the propagation times of the microwave signals from the satellites to the receiver will introduce errors into the calculations performed by receiver. For example, if the signal from each satellite is delayed by 100 ns, then the time calculated by the receiver will also be delayed by 100 ns.
The microwave signals transmitted by the GPS satellites are significantly affected by their passage through the ionosphere. The ionosphere is a layer of the atmosphere at an elevation of 150-1,000 km that contains free electrons generated by ionizing radiation from the sun. The distribution and density of the free electrons at a given point in the ionosphere varies strongly with the time of day, the time of year, and the state of the solar sun-spot cycle. There is also a significant unpredictable variation due to fluctuations in solar activity. The ionosphere can typically delay the microwave signals from the satellites by up to 100 ns. The time signals generated by the GPS satellites are highly accurate and stable. Thus, a GPS receiver can potentially be used as a simple, low-cost local time standard. However, the unpredictability and variability of the ionospheric delay limits the accuracy and stability of the time the can be generated by a conventional GPS receiver.
GPS receivers exist that are capable of detecting microwave signals at both of the two widely-separated frequencies, L1 and L2, transmitted by the GPS satellites. Such receivers remove the effect of the ionospheric delay using the fact that the ionosphere imposes a group delay on the microwave signals that varies inversely with the square of the carrier frequency. However, the L2 ranging signal is currently encrypted and cannot easily be decoded by users who are not qualified by the United States Department of Defense. Some two-frequency receivers are available for non-qualified users, but they are considerably more expensive, and somewhat less reliable, than single-frequency receivers. Accordingly, many users have single-frequency receivers that can receive only the L1 signal.
Single-frequency receivers usually include a correction for the ionospheric delay based on an ionosphere model that is built into the GPS system. This model is expected to remove about 50% of the ionosphere effect, on average. Since the parameters of the model are estimated in advance and are then transmitted to the GPS satellites, it cannot anticipate day-to-day random fluctuations, and cannot be completely accurate. Alternatively, various organizations make detailed and accurate models of the ionosphere based on GPS observations. However, these models are not available simply, or in real-time.
What is needed, therefore, is a way of autonomously measuring the ionospheric delay of a signal transmitted by a satellite. The ability to measure the ionospheric delay autonomously would enable a single-frequency GPS time receiver to generate a highly-accurate time signal that is not subject to the inaccuracies that result from the limited performance of the conventional built-in ionosphere model.
The invention provides a method for measuring, autonomously and substantially in real-time, an ionospheric delay of a GPS signal transmitted by a first satellite having an obliquity. In the method, a code range, a carrier phase range and a satellite elevation angle for each of at least two satellites are received. For each of the at least two satellites, a code-phase divergence between the code range and the carrier phase range is calculated, an obliquity is calculated from the satellite elevation angle, and time derivatives of the code-phase divergence and of the obliquity are calculated. A zenith delay is calculated from the obliquities, the time-derivatives of the obliquities and the time-derivatives of the code-phase divergences of the at least two satellites. The ionospheric delay is then calculated by multiplying the obliquity of the first satellite and the zenith delay.
A code range, a carrier phase range, a satellite elevation angle and additionally a satellite azimuth angle may be received for each of at least three satellites. In this case, for each of the at least three satellites, an offset longitude and an offset latitude are calculated from the elevation angle and the azimuth angle, and a time derivative of the offset longitude and a time derivative of the offset latitude are calculated. The zenith delay is calculated from the obliquities, the time-derivatives of the obliquities and the time-derivatives of the code-phase divergences of the at least three satellites, and additionally from the offset longitudes, the offset latitudes, the time derivatives of the offset longitudes and the time derivatives of the offset latitudes of the at least three satellites. A derivative of the zenith delay with offset longitude and a derivative of the zenith delay with offset latitude are calculated from the obliquities, the time-derivatives of the obliquities and the time-derivatives of the code-phase divergences, the offset longitudes, the offset latitudes, the time derivatives of the offset longitudes and the time derivatives of the offset latitudes of the at least three satellites. The ionospheric delay is then calculated from the zenith delay, the derivative of the zenith delay with offset longitude and the derivative of the zenith delay with offset latitude and additionally from the obliquity, the offset longitude and the offset latitude of the first satellite.
The autonomous ionospheric delay measuring method may be used in a method for generating, from GPS signals transmitted by a single satellite or by a set of P satellites, a corrected receiver clock bias signal corrected for ionospheric delay. The ionospheric delay measured by the autonomous ionospheric delay measuring method is used to correct the receiver clock bias signal. This correction enables the difference, characterized by the receiver clock bias, between a receiver time signal and the GPS system clock to be known more accurately.
The invention also provides an ionospheric delay measuring apparatus that operates autonomously and substantially in real-time to measure an ionospheric delay of a GPS signal transmitted by a first satellite and received by a GPS front end capable of receiving a GPS signal from each of at least two satellites and configured to calculate, from each GPS signal, a code range, a carrier phase range and a satellite elevation angle. The apparatus comprises a divergence and obliquity module for each of the at least two satellites, a zenith delay module and an ionospheric delay module.
Each divergence and obliquity module is connected to receive from the GPS front end the code range, the carrier phase range and the satellite elevation angle of one of the at least two satellites. Each divergence and obliquity module includes a code-phase divergence module, an obliquity module and time derivative modules. The code-phase divergence module is structured to calculate a code-phase divergence between the code range and the carrier phase range. The obliquity module is structured to calculate an obliquity from the satellite elevation angle. The time derivative modules are structured to calculate a time derivative of the code-phase divergence and a time derivative of the obliquity.
The zenith delay module receives the obliquities, the time-derivatives of the obliquities and the time-derivatives of the code-phase divergences of the at least two satellites and is structured to calculate a zenith delay from these quantities.
The ionospheric delay module calculates the ionospheric delay of the first satellite by multiplying the obliquity of the first satellite and the zenith delay.
The GPS front-end may be additionally capable of receiving a GPS signal from each of at least three satellites and may be structured additionally to calculate an azimuth angle from each GPS signal. In this case, the apparatus may include a divergence and obliquity module for each of the at least three satellites. Each divergence and obliquity module is additionally connected to receive the azimuth angle for one of the satellites from the GPS front end, and additionally includes an offset module and additional time derivative modules. The offset module calculates an offset longitude and an offset latitude from the elevation angle and the azimuth angle. The additional time derivative modules are structured to calculate a time derivative of the offset longitude and a time derivative of the offset latitude. In such apparatus, the zenith delay module is structured to calculate the zenith delay, a derivative of the zenith delay with offset longitude and a derivative of the zenith delay with offset latitude from the obliquities, the time-derivatives of the obliquities and the time-derivatives of the code-phase divergences of the at least three satellites, and additionally from the offset longitudes, the offset latitudes, the time derivatives of the offset longitudes and the time derivatives of the offset latitudes of the at least three satellites. Finally, the ionospheric delay module is structured to calculate the ionospheric delay from the zenith delay, the derivative of the zenith delay with offset longitude and the derivative of the zenith delay with offset latitude and additionally from the obliquity, the offset longitude and the offset latitude of the first satellite.
The autonomous ionospheric delay measuring apparatus may form part of a GPS time receiver in which the ionospheric delay measured by the autonomous ionospheric delay measuring apparatus is used to correct the receiver clock bias of the GPS time receiver.